Presently, the commercial bitumen extraction process for mined oil sands is Clark hot water extraction technology or its variants that use large amounts of water and generate a great quantity of wet tailings. Part of the wet tailings becomes mature fine tailings (MFT), which contain approximately 30% fine solids and are a great challenge for tailings treatment. In addition, certain “problem” oil sands, often having high fines content, yield low bitumen recoveries in the water-based extraction process. This leads to economic losses and environmental issues with bitumen in wet tailings.
An alternative to water-based extraction is solvent extraction of bitumen from mined oil sands, which uses little or no water, generates no wet tailings, and can potentially achieve higher bitumen recovery than the existing water-based extraction, especially for the aforementioned problem oil sands. Therefore, solvent extraction is potentially more robust and more environmentally friendly than water-based extraction.
The majority of solvent extraction processes taught in the prior art use a single solvent or a solvent mixture having a fixed composition throughout the process. This solvent may be a light solvent with a typical boiling range of 36-110° C., an intermediate solvent with a typical boiling range of 66-205° C., or a heavy solvent with a typical boiling range of 177-343° C. Examples of the light solvents are C5-C6 (U.S. Pat. No. 4,347,118 and U.S. Pat. No. 4,752,358), cyclohexane (U.S. Pat. No. 4,189,376), toluene (U.S. Pat. No. 4,416,764), heptane/toluene mix (U.S. Pat. No. 4,448,667), oxygenated C2-C4 (U.S. Pat. No. 4,929,341) and chlorinated C1-C2 (U.S. Pat. No. 4,532,024 and U.S. Pat. No. 6,207,044). Use of pure solvents, such as cyclohexane, toluene, oxygenated solvents or chlorinated solvents is not practical since they are usually not available in large quantities to oil sand bitumen producers. The readily available solvent is light naphtha (essentially mixed aliphatic C5-C7), but it is not compatible with bitumen. Asphaltene precipitates out of bitumen after mixing with this solvent, contributing to lower hydrocarbon recovery. Asphaltene precipitation in a large quantity may also cause equipment fouling and plugging, and oily dry tailings unsuitable for disposal.
Alternately, an intermediate solvent such as naphtha may be used for solvent extraction of bitumen (Canadian Patent No. 1,190,877 and U.S. Pat. No. 5,534,136). Naphtha is generally compatible with bitumen owing to its light aromatic components such as benzene, toluene, ethylbenzene and xylenes (BTEX), and/or heavy fractions with boiling points near 200° C. However, BTEX are considered toxins in soil even at ppm levels. In commercial solvent extraction operations, it would be unavoidable to include trace amounts of solvent in the dry tailings. Environmental regulations may forbid the use of any solvent containing significant amounts of BTEX in this application. The heavy fractions in naphtha make solvent recovery difficult. These fractions are considered volatile organic compounds (VOC). Regulations for VOC emissions may limit the residual light and intermediate solvent content less than 300 mg/kg of dry tailings for oil sands operations. To meet the VOC requirement, large energy input is needed to recover solvent fractions from spent solids at around 200° C. This usually makes the process uneconomical and increases the greenhouse gas emissions as well.
Alternately, a heavy solvent may be used for solvent extraction of bitumen. Examples of the heavy solvent include kerosene (U.S. Pat. No. 4,094,781) and diesel (Canadian Patent No. 1,048,432). The main problem with the heavy solvents is the poor solvent recovery from spent solids. To fully recover the heavy solvents, energy-intensive operations such as retorting or coking the spent solids are required. Energy used to heat the spent solids in these operations is usually unrecoverable, making the process uneconomical.
In addition, the use of any light or intermediate solvent poses fire hazard during the initial contact with oil sands in a vessel that is not adequately purged with an inert gas. Effectively purging such a vessel is a challenge due to the sticky nature of oil sands that may not allow the use of air locks for the feed. Hence, a process using any single solvent would be hindered by one or several of the aforementioned difficulties.
It has been suggested that using two solvents sequentially may overcome some of these problems. For example, a light aromatic solvent (Canadian Patent No. 2,582,078) or naphtha (U.S. Patent Application No. 2010/0032348) is used first for bitumen extraction, which causes no asphaltene precipitation. Subsequently, a second volatile solvent (C3-C5) is used for the extraction of the first solvent from the spent solids. Since bitumen is mostly removed with the first solvent, the second solvent could be a poor bitumen solvent such as liquefied propane or butane without causing significant asphaltene precipitation. However, as mentioned above, the use of BTEX-containing solvents could be problematic due to the soil toxicity issue. Fire hazard during the initial contact is also a problem.
Alternately, the first solvent can be a heavy, aromatics-rich, high-flash point solvent such as a light gas oil (LGO) (U.S. Pat. No. 3,131,141 and U.S. Pat. No. 3,117,922). It does not cause asphaltene precipitation, does not contain BTEX, and does not pose fire hazard at a typical process temperature of 20-80° C. After bitumen removal, a second light solvent is used for the extraction of the first heavy solvent from the solids. Solvent recovery from spent solids would be relatively easy after the light solvent replacement. However, separating viscous bitumen-LGO solutions from solids is a challenge. A very high LGO/bitumen ratio may be required for the separation. Since LGO requires higher temperature (over 300° C.) to distill and recycle, a high LGO/bitumen ratio would likely make the process uneconomical.
All of the aforementioned processes using two solvents have one feature in common, i.e. the solvent switch (from the first to the second solvent) occurs after the near complete extraction of bitumen. In addition to the problems mentioned above, one disadvantage of the processes with this feature is that the total solvent demand is usually twice as high as that of a single-solvent process since these dual-solvent processes are essentially two separate extractions in series. The higher solvent demand greatly increases the cost of solvent storage, handling and recycle.
U.S. Pat. No. 4,389,300 teaches feeding oil sands, presumably dry-crushed, into a single vertical column containing both countercurrent heavy solvent wash and light solvent wash at different depths. The light solvent after countercurrent wash was not completely withdrawn from the column and was allowed to mix with the heavy solvent to the point of initial contact with oil sands. Therefore, the light solvent also contributed to the bitumen extraction. This extraction scheme may reduce the total solvent demand, but the presence of the light solvent poses fire hazard at the initial contact. Additionally, the ratio of the two solvents cannot be precisely controlled or varied in various locations of a column without discrete stages. Thus, the proportion of light solvent could be either too small, thereby failing to lower the viscosity of the bitumen solution significantly, or could be too large, thereby causing asphaltene precipitation. Additionally, in a commercial-scale operation, it is difficult to crush dry oil sands to a lump size amenable to extraction without the aid of solvent or hot water.
All prior art processes were proposed for bulk oil sands without ore segregation. Due to the inherent complexity of bitumen-solids separation and solvent recovery in solvent extraction, all solvent extraction processes are uneconomical compared with the existing water-based extraction process if they are used for bulk oil sands. However, certain problem oil sands that do not yield high bitumen recoveries in water-based extraction may have higher bitumen recoveries in solvent extraction. Hence, segregating problem oil sands from bulk oil sands for solvent extraction may bring in higher economical return. This opportunity was not explored in the prior art.
In summary, none of the prior art solvent extraction processes can resolve all of the following issues:
1. Fire hazard at the initial contact of solvent with oil sands;
2. All types of solvents except for light solvents contain heavy fractions that pose a challenge in solvent recovery from spent solids;
3. BTEX in light aromatic solvents or naphtha cause toxicity issue in dry tailings disposal;
4. Light solvents that are easy to recover from spent solids and contain no BTEX cause asphaltene precipitation from bitumen;
5. Attempts to solve the above issues by using two solvents sequentially encounter solid/liquid separation problem and issues with higher solvent demand and operating cost;
6. Complete solvent recovery from spent solids to meet the environmental requirements, e.g. VOC limit, involves energy-intensive operations that increase the green-house gas emissions;
7. Being inherently more complicated, all solvent extraction processes appear uneconomical compared with the existing water-based extraction process if they are used for bulk oil sands without ore segregation.
There is a need for a solvent extraction process that is safe, operable, economical and environmentally friendly.